This invention relates to knee endoprostheses, and, more particularly, to femoral components for knee endoprostheses.
Femoral components having a smooth articulating surface defined by a plane curve rotated about a plurality of axes are known in the prior art. A widely accepted polycentric femoral design in the field of endoprosthetics is disclosed in U.S. Pat. Nos. 4,309,778, 4,340,978 and 4,470,158 which issued to the inventor of the subject application as a co-inventor, and shown in FIGS. 1 and 2. The disclosures of U.S. Pat. Nos. 4,309,778, 4,340,978 and 4,470,158 are incorporated by reference herein for thorough and extensive descriptions of the prior art femoral design. To briefly summarize, referring to FIGS. 1 and 2, femoral component 100 has an outer bearing surface 102 for articulation with a thermoplastic bearing insert and, optionally, a patella component. The bearing surface 102 defines two condyles on the femoral component 100, each being arcuately formed in two coordinate directions. As shown in FIG. 1, a plane curve "F" is defined by arcs K1, K2, and K3, generated respectively about center points H1, H2, and H3 with respective radii A1, A2, and A3 and tangent lines 107, 108 blending respectively arcs K2 and K3, and K1 and K3. Referring to FIG. 2, the plane curve "F" is rotated respectively about axes C1-C4 to respectively generate segments of surfaces of revolution S1-S4 from the anterior portion of the femoral component to the posterior portion. The largest segment S2 is referred to as the primary load bearing surface segment, since the portions of the femoral component corresponding to the segments S2 bear the majority of load during use. The radius used to define the segment S2 from the axis C2, for each of the condyles, is equal to the radius A1, which in turn is equal to the radius A2. It is thus apparent, the segment S2 of each of the condyles of the femoral component is spherically generated about the axis C2. The arcuate length of the segment S2 allows congruent articulation between the femoral component and a bearing insert during use over the range from full extension to approximately 32.degree. of flexion. The bearing insert used with this femoral component is formed with two bearing surfaces, each corresponding to a condyle, and each dimensioned to the spherical dimensions of the segments S2. It can be appreciated, that during the majority of activity by a wearer of the prior art femoral component and bearing insert combination, congruent contact is maintained between the condyles and the bearing surfaces of the bearing insert over the arcuate length of the segment S2. As such, area contact is developed and maintained therebetween during normal activity of a wearer of the prosthesis, such as walking and standing, and stress loading is minimized. Outside the range of congruent contact, incongruent contact is generated resulting in line or point contact. It should be noted that the knee prosthesis described above operates well under incongruent articulation.
Although there are many advantages to the above-described femoral component, there still exists the need for a femoral component which can maintain congruency with a bearing insert over a larger range of flexion than in the prior art.